A geothermal power plant started operating in Southern Reykjanes Peninsula, Iceland in May 2006. The power plant utilises geothermal water ejected from boreholes in the region to generate electricity. In the following months the region started to subside due to the ejection. A similar scenario had previously been observed in Svartsengi power plant further north in the peninsula after it started electric- and heat production in 1976. The main purpose of this thesis is to measure this subsidence with GPS surveying equipment, collect older data from previous GPS campaigns and put into perspective with the new data by creating a continuous time series. Measurements were carried out between 30th March and 7th May at ten locations in the vicinity of the power plant in Reykjanes. A semi-continuous GPS station was operated near the plant during the campaign. Another semi-continuous station was installed south of Sýrfell in collaboration with the power plant's owner, HS Power. The new station is intended to operate continuously to monitor ongoing changes in the area. In the years prior to the electrical production a relatively insignificant subsidence, 1-5 mm/yr, had occurred in the area. Following the production the subsidence increased quite rapidly and during the time period of 2006-2009 a maximum rate of 40 mm/yr was observed in stations close to the boreholes.